Preheat broiler.
Lay toast, buttered sides up, on rack in broiler pan; top with asparagus and turkey.
Mix mayonnaise, lemon juice, and pimiento; fold in egg whites and spread evenly over turkey, right to edges of toast.
Broil 6-7" from heat about 5 minutes until flecked with brown and puffy or, if you prefer, bake 3-5 minutes at 450° F. instead of broiling.